1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly, to a method for a handoff of a medium rate data call in a mobile communication system.
2. Description of the Background Art
Generally, a handoff in a mobile communication system is a function of keeping a call in service from being disconnected when a mobile station moves from the current service area to other adjacent service area.
FIG. 1 is a block diagram illustrating the construction of a general mobile communication system.
As illustrated therein, the mobile communication system includes: a mobile station 1 operated during movement or while stopping at a undetermined spot; a base station subsystem(BS) 10 for processing a call from the mobile station 1 via a radio interface with the mobile station 1; and a mobile switching center(MSC) 20 switching the call transmitted from the base station 10 to a corresponding subscriber. The base station 10 includes: a BTS(base station transceiver subsystem) 11 for interfacing with the mobile station 1; and a BSC(base station controller) 12 for processing a call transmitted from the BTS 11 and controlling the BTS 11.
Handoffs in a mobile communication system for providing a voice service according to the IS-95 and IS-95A standards include the ADD Threshold(T—ADD), DROP Threshold(T—DROP), COMPARISON Threshold(T—COMP), and DROP TIMER Threshold(T—TDROP).
The T—ADD is a minimum pilot strength capable of maintaining an available speech channel, e.g., a threshold of a pilot strength that is used by the mobile station 1 in order to demand a handoff. The T—DROP is a pilot strength which is required for the release of an unavailable speech channel. The T—COMP is a threshold used to demand a handoff of the BTS in a candidate set. The T—TDROP is a timer used to drop the handoff BTS lower than the T—DROP.
The mobile station 1 manages the BTS by dividing it into an active set, candidate set, and neighbor set when it performs a handoff. The active set is a set of BTSs having a pilot strength more than T—ADD and currently providing a service to the mobile station 1. The candidate set is a set of candidate BTSs having a pilot strength more than T—ADD, but currently preparing to provide a service to the mobile station 1. The neighbor set is a set of BTSs neighboring to the BTSs belonging to the active set and the candidate set.
FIG. 2 is a diagram illustrating a handoff method when a voice service is provided according to the IS-95 and IS-95A standards in a mobile communication system.
As illustrated therein, the mobile station 1 stores the T—ADD, T—DROP, T—COMP, and T—TDROP which are a handoff decision element contained in a paging message transferred when being paged by the base station 10.
After a call is set up via a serving base station 10, the mobile station 1 measures the pilot strength of the serving base station 10 and the pilot strength of neighbor or remaining BTSs(base station subsystems) classified as the neighbor set.
Thereafter, when the pilot strength of the BTS classified as the neighbor set becomes more than T—ADD, the mobile station 1 transfers a pilot strength measurement message (PSMM) having the pilot information and pilot strength of the BTSs in the neighbor set and the pilot strength thereof to the serving base station 10, and includes the BTS generating this pilot signal into the neighbor set (a).
The serving base station 10 analyzes the transferred PSMM, and transfers a handoff direction message containing this pilot signal to the mobile station 1 (b).
The mobile station 1 includes the BTS generating this pilot signal into the active set, and reports the completion of the ADD handoff to the base station 10 (c).
Afterwards, when the pilot strength of the serving base station 10 classified as the existing active set becomes less than T—DROP, the mobile station 1 drives the T—TDROP (d). When the driven T—TDROP is finished, the mobile station 1 transfers the PSMM to the serving base station 10 (e).
The serving base station 10 analyzes the transferred PSMM and transfers a handoff direction message containing a pilot information of the corresponding BTS which satisfies the active set with the pilot strength, to the mobile station 1 (f). The mobile station 1 moves the serving base station 10 from the active set to the neighbor set according to the handoff direction message, and reports the completion of the DROP handoff (g).
In this way, in case of providing a voice service according to the IS—95 or IS-95A standards, the handoff method has a problem that a handoff is frequently occurred according to a radio state, because the PSMM is transferred to the base station when the pilot strength of a neighbor BTS becomes more than T—ADD. In addition, there is a problem that system load is increased and sound quality is degraded due to the frequent occurrence of allocation/deadlocation of resources.
In case of implementing a data service of a medium data rate(64 Kbps) in a conventional mobile communication system in which such 8 Kbps and 13 Kbps voice services are provided, physical radio resources are additionally required in order to increase a data rate. An additional allocation of radio resources for the implementation of a medium rate data service results in the more frequent occurrence of allocation/deadlocation of resources, and thus an improved handoff method is proposed in the IS-95B standard in order to compensate these problems.
In other words, four fields are added to a conventional system parameter and handoff direction message. The four fields includes SOFT—SLOPE, ADD—INTERCEPT, DROP—INTERCEPT, and P—REV. The SOFT—SLOPE is a parameter for deciding whether the mobile station performs a handoff procedure according to the IS-95 and IS-95A standards or performs a handoff procedure according to the IS-95B standard. If the SOFT—SLOPE field has a value of “0”, the mobile station conforms to the handoff procedure according to the IS-95 and IS-95A standards, or if it has other values, the mobile station conforms to the handoff procedure according to the IS—95B standard.
When the mobile station receives a paging message containing the four files from the BTS, it stores the same.
Afterwards, in the case that a data call is set up and the SOFT—SLOPE is not “0”, when the pilot strength of an arbitrary BTS belonging to the neighbor set becomes more than T—ADD, the mobile station computes a new dynamic threshold using the SOFT—SLOPE and ADD—INTERCEPT and includes the arbitrary BTS into the candidate set. The mobile station judges if the pilot strength of the BTS included in the candidate set is larger than the dynamic threshold.
If the pilot strength of the BTS is larger than the dynamic threshold, the mobile station transfers a PSMM containing the corresponding pilot information and pilot strength to the BTS. The subsequent ADD handoff procedure is achieved in the same way as the handoff procedure conforming to the IS-95 and IS-95A standards.
In addition, in case of DROP of the BTS belonging to the active set, the mobile station computes a new dynamic threshold using the SOFT—SLOPE and DROP—INTERCEPT The mobile station compares the pilot strength of the BTS belonging to the active set with the computed dynamic threshold. As the result of the comparison, if the pilot strength of the BTS belonging to the active set is lower than the dynamic threshold, the T—TDROP is driven.
If the pilot strength of the BTS is lower than the dynamic threshold after the driven T—TDROP is finished, a PSMM containing the corresponding pilot strength and corresponding pilot information to the base station. The subsequent DROP handoff procedure is achieved in the same way as the handoff procedure conforming to the IS-95 and IS-95A standards.
In case of the ADD handoff, the SOFT—SLOPE and ADD—INTERCEPT are set such that the dynamic threshold using the SOFT—SLOPE and DROP—INTERCEPT is higher than T—ADD in general. Also in case of the DROP handoff, the SOFT—SLOPE and DROP—INTERCEPT are set such that the dynamic threshold using the SOFT—SLOPE and DROP—INTERCEPT is higher than T—DROP. In addition, the SOFT—SLOPE, ADD—INTERCEPT and DROP—INTERCEPT can be reset by a system operator according to a radio environment.
Such a method for a handoff of a medium rate data call according to the IS-95B standard has an effect that a frequent handoff is prevented by setting a dynamic threshold higher than T—ADD and T—DROP. On the other hand, there occurs a problem that, since the mobile station must compute a dynamic threshold for the ADD handoff and a dynamic threshold for the DROP handoff each time, the load of the mobile station is increased.
In addition, since a voice service according to the IS-95 and IS-95A standards is transferred via FCH along with data in voice, only the FCH is considered during handoff. However, in case of a medium rate data service, a signal is transferred via a fundamental channel(FCH), and data is transferred via a supplemental channel(SCH). Thus, in the method for handoff of a medium rate data call according to the IS-95B standard in the conventional art, FCHs and SCHs are allocated to all BTSs of the active set, and these two kinds of channels are dropped at the same time without discriminating the FCH from the SCH. Therefore, in the method for handoff of a medium rate data call according to the IS-95B standard in the conventional art, there arises a problem that, since the FCHs and SCHs are allocated to all BTSs in the active set, the utilization efficiency of an available SCH of the base station is degraded.